Semiconductor-based audio power amplifiers have been dominating the audio market since late 1960's. Advantages of employing semiconductor devices in audio signal amplification include compact size, cost effective power handling and high output power. However, the sound quality of semiconductor audio amplifiers is different from vacuum tube audio amplifier, and many audio consumers prefer the sound of amplifiers using vacuum tubes to those using semiconductors. In the high-end audio market, vacuum tube audio systems have gained a very favourable market share since its return in the late 1980's.
In the high-end audio applications, balanced topologies are often used in semiconductor amplifiers, both power and low-level (i.e., pre-amp and line amp) amplifiers. The advantage of employing balanced topology is that common mode noises and distortions can be cancelled out at the output. The result is a lower noise and lower distortion amplifier compared with amplifiers not employing any balanced topology. However, there has been very little development on vacuum tube balanced power amplifiers. Single-ended input is commonly used in vacuum tube amplifiers.
At present, there is a balanced topology found in vacuum tube power amplifier design. This balanced topology can be explained by viewing a power amplifier as having three separate amplifying stages, namely input stage (also called “first stage”), second stage (also called “driver stage”) and output power stage (also called “output stage”). In such a topology, a pair of feedback loops is connected from the output power stage to the input stage.
For example, a functional block diagram and a circuit implementation of a conventional balanced amplifier using one pair of feedback paths is shown in FIGS. 7 and 8, respectively. A functional block diagram and a circuit implementation of a conventional complete balanced audio power amplifier in a push-pull configuration is illustrated in FIGS. 9 and 10.
From FIGS. 9 and 10, it can be easily seen that one single pair of feedback loops are connected between the output stage and the input stage. The applied feedback will generally improve the performance of the power amplifiers, i.e., lower output impedance, broader bandwidth and lower distortion. However, it is well known that if too much feedback is applied, it will render instability to the amplifier.
As is well known from the feedback theory, feedback will reduce distortion and output impedance, and increase the bandwidth of the overall power amplifier. Although these are all desirable features, as stated above, applying too much feedback will create instability to the amplifier, whereas applying too little feedback will not obtain the desired results. A designer thus always has difficulty in optimizing the overall power performance by using a single pair of feedback loops.
It is thus an object of the present invention to provide a balanced amplifier in which the aforesaid shortcomings are mitigated or at least to provide a useful alternative. In particular, the present invention provides a new balanced amplifier topology that can be employed in vacuum tube power amplifier applications for improving performance.
According to the present invention, there is provided a balanced amplifier comprising at least an input stage connected with a second stage, wherein said input stage is adapted to output signals to said second stage, wherein said first stage includes a first differential amplifier adapted to receive balanced input signals and is provided with two pairs of feedback loops, wherein at least one said pair of feedback loops are adapted to feedback part of signals outputted by said second stage to said first differential amplifier, and wherein said second stage includes a second differential amplifier adapted to amplify signals received from said first stage and is adapted to maintain said balanced signals.